The application is very similar to Transmission Remote GUI in terms of Bittorrent functionality, however, Transmission Remote GTK integrates better with the GNOME desktop (since it's a GTK app; sadly it's GTK2 only), comes with torrent added/downloaded notifications as well as tray / Ubuntu AppIndicator support.

What is this tool for? Well, if you've set up Transmission Daemon on a server (it doesn't really has to be a server but anyway), you can use Transmission Remote GTK to control it remotely. For how to set up Transmission Daemon on Ubuntu / Debian, see our old (should still work) post: Setting Up Transmission-Daemon in Ubuntu / Debian.